Auxiliary catalytic electrodes

Recombination of Hydrogen and Oxygen to Water on Auxiliary Catalytic Electrodes... 

This method is based on electrochemical reduction of oxygen in a closed oxygen cycle (COC) and electrochemical oxidation of hydrogen in a closed hydrogen cycle within the cell. The above electrochemical reactions proceed on auxiliary catalytic electrodes partially immersed in the electrolyte and connected by means of electronic devices (ED) to the negative electrode (for oxygen reduction) and to the positive electrode (for hydrogen oxidation), respectively. 

A schematic diagram of a lead—acid cell with auxiliary catalytic electrodes is presented in Fig. 14.3. 

The auxiliary electrodes intended to facilitate the reduction of oxygen comprise of, for example, carhon—teflon porous mass catalyzed with silver [6,7] or phthalocyanine [8]. Ruetschi and Ockermann have established that both hydrogen oxidation and oxygen reduction can proceed on the same auxiliary electrode [9]. In our institute, we developed auxiliary catalytic electrodes with tungsten carbide (WC) or a mixture of WC and active carbon as catalyst [10—12], Figure 14.4 presents the volt-ampere curves of the hydrogen and oxygen reactions on partially immersed auxiliary electrodes with different catalysts WC alone, or WC plus active carbon (WC-I-C), or platinum (Pt) [10-12]. 

The data in the figure indicate that the catalytic activity of the WC-hC electrode towards oxygen reduction is commensurate with its activity towards hydrogen oxidation. Active carbon added to the WC enhances the electrode s catal3rtic activity with regard to both reactions. Electrodes catalyzed with Pt are twice more efficient than die WC and WC+C electrodes, but they are much more expensive as well. Hence, we focused our attention on the WC-fC catalytic electrodes. Further in this chapter, we will discuss the behaviour of WC-based auxiliary catalytic electrodes during operation of 12 V/82 Ah cell. 

Tests of high-capacity stationary cells with auxiliary catalytic electrodes were conducted at a number of battery plants, but on grounds of certain technical and economical considerations, this method of recombination of hydrogen and oxygen has not found wide practical application in the battery industry. 

The electrons produced in the conduction band as a result of illumination can participate in cathodic reactions. However, since in n-type semiconductors the quasi-Fermi level is just slightly above the Fermi level, the excited electrons participating in a cathodic reaction will almost not increase the energy effect of the reaction. Their concentration close to the actual surface is low hence, it will be advantageous to link the n-type semiconductor electrode to another electrode which is metallic, and not illuminated, and to allow the cathodic reaction to occur at this electrode. It is necessary, then, that the auxiliary metal electrode have good catalytic activity toward the cathodic reaction. 

NEMCA effect — The term NEMCA is the acronym of Non-faradaic Electrochemical Modification of Catalytic Activity. The NEMCA effect is also known as electrochemical promotion (EP) or electropromotion. It is the effect observed on the rates and selectivities of catalytic reactions taking place on electronically conductive catalysts deposited on ionic (or mixed ionic-electronic) supports upon application of electric current or potential (typically 2 V) between the catalyst and a second (counter or auxiliary) electrode also deposited on the same support. The catalytic reactants are usually in the gas phase. 

The cathode formulations further employ metallic catalysts to promote the recombination of hydrogen gas generated within the zinc anode at very low (0.025 %) or zero mercury content [41,42,43]. The hydrogen scavenger can form a discrete auxiliary electrode in electronic contact with the cathode or can be introduced simply as a cathode additive in a ratio of 30/70 in the cathode preparation process. A very reliable catalytic material even at elevated temperatures (65°C) is Ag20. The auxiliary electrode may be an annulus or a disk of similar... 

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